EEE Vaccination Timing Critical for Protection

Vaccinating at the proper time of the year against Eastern equine encephalitis (EEE) is critical to protecting horses from the sometimes-fatal mosquito-borne disease, according to Judy Marteniuk, DVM, MS, associate professor in Equine Medicine and Extension at Michigan State University. As of Aug. 19, 18 cases have been confirmed, with seven pending lab results.

Eastern equine encephalitis cannot be transmitted from horse to horse, but rather is spread by mosquitoes that serve as the vectors, or carriers, that transmit the disease to horses. Horses vaccinated after late March should be protected during mosquito season, but those that were vaccinated prior to March should receive an EEE booster, Marteniuk noted.

Broodmares that are set to foal early in 2011 that were vaccinated early this spring should receive a booster vaccine in order to provide protection to their foals (the antibodies will be passed to the foals through the mares' colostrum). Additionally, owners who have been vaccinating annually without keeping protection during mosquito season in mind should booster their horses, Marteniuk said. The vaccine should be effective for six to eight months, she noted.

"There have been cases of horse fatalities in Michigan because the owner was vaccinating for EEE in the fall, and the horses were not protected during the entire mosquito season," Marteniuk said. "It's important that owners consult with their veterinarians to ensure proper timing of vaccines and other health protocols."

According to veterinarians, most horses that have developed the disease in Michigan were not vaccinated. Marteniuk noted that Michigan horse owners can vaccinate their unvaccinated horses now to protect their horses for the remainder of the season.

Karen Waite, MS, equine specialist with MSU Extension added "If you have a horse and are unsure of its vaccination history, you should revaccinate them to be sure they are protected."

In addition, Marteniuk advised owners of horses that have not been vaccinated in the past one to two years: Vaccinate these animals two weeks after the initial EEE vaccination, rather than waiting the routine vaccination protocol of three to four weeks. She pointed out that horses could still become infected in the first seven to 10 days post-vaccination.

Mosquitoes can also transmit EEE to humans. Because of the high mortality rate among infected horses and people, EEE is regarded as one of the most serious mosquito-borne diseases in the United States.

The Michigan Departments of Community Health (MDCH) and Agriculture (MDA) encourage people to take precautions when EEE has been identified in horses or people in their area. Recommended measures include applying mosquito repellent, repairing or replacing broken window screens on buildings where mosquitoes could enter, and draining standing water sources that could become mosquito-breeding grounds.

In a joint news release from the Michigan Department of Agriculture and the state's Department of Community Health, Steven Halstead, DVM, MS, Michigan state veterinarian, said, "A simple vaccination will protect your animal from these often-fatal illnesses, and routine measures to reduce mosquito exposure and eliminate mosquito habitats around the home and farm will help protect people, horses and other livestock."

To find out more information about EEE and vaccinations or to develop a horse health program, contact your local veterinarian.

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